Actuation is essentially a mechanism whereby a device is turned on or off, or is adjusted or moved by converting various types of energies such as electric energy or chemical energy into mechanical energy. Mechanical energy can be stored as elastic energy in a material or a device, or can be used to produce useful mechanical work, which is defined as the product of stress and strain. Thus a useful measure of the potential for actuation of a given material or device is the actuation energy density (energy per unit volume). The actuation energy density is also useful for distinguishing the capabilities of different actuation methods. The specific (or gravimetric) energy is readily obtained from the energy density knowing the density of the materials or device. While the “free strain,” or strain produced under zero or nearly zero stress conditions, is sometimes used to characterize actuators or actuation materials, this is an inadequate measure of actuation capability since no mechanical work is done. Thus, the capability for mechanical work can only be known when the strain produced against a known mechanical stress, or the stress produced under known conditions of strain, are known.
Different types of actuators are categorized by the manner in which energy is converted. For instance, electrostatic actuators convert electrostatic forces into mechanical forces. Piezoelectric actuators use piezoelectric material to generate kinematic energy. Electromagnetic actuators convert electromagnetic forces into kinematic energy using a magnet and coil windings.
Actuation, in theory, would find application in the production of adaptive and morphing structures, though practically such an application has not produced ideal results. Piezoelectric actuation provides high bandwidth and actuation authority but low strain (much less than 1% typically), and requires high actuation voltages. Shape memory alloys (SMAs), magnetostrictors, and the newly developed ferromagnetic shape-memory alloys (FSMAs) are capable of larger strain but produce slower responses, limiting their applicability. Actuation mechanisms that are based on field-induced domain motion (piezos, FSMAs) also tend to have low blocked stress. All the above actuation methods are based on the use of active materials of high density (lead-based oxides, metal alloys), which negatively impacts weight-based figures of merit. Thus there is currently a great need for a technology capable of providing high actuation energy density, high actuation authority (stress), large free strain, and useful bandwidth.
Certain methods of actuation using electrochemistry have previously been described. For example, K. Oguro, H. Takenaka and Y. Kawami (U.S. Pat. No. 5,268,082) have described using surface electrodes to create ion motion under applied electric field across an ion-exchange membrane resulting in deformation of the membrane. W. Lu, B. R. Mattes and A. G. Fadeev (U.S. Patent Application No. 2002/0177039) have described using ionic liquid electrolytes in conjugated polymers to obtain dimensional change. R. H. Baughman, C. Cui, J. Su, Z. Iqbal, and A. Zhakidov (U.S. Pat. No. 6,555,945) have used double-layer charging of high surface area materials to provide for mechanical actuation. D. A. Hopkins, Jr. (U.S. Pat. No. 5,671,905) has described an actuator device in which electrochemically generated gas pressure is used to provide for mechanical motion. H. Bauer, F. Derisavi-Fard, U. Eckoldt, R. Gerhrmann and D. Kickel (U.S. Pat. No. 5,567,284) have similarly used electrochemically-produced gas pressure in a pneumatic actuation device. G. M. Spinks, G. G. Wallace, L. S. Fifield, L. R. Dalton, A. Mazzoldi, D. De Rossi, I. I. Khayrullin, and R. H. Baughman (Advanced Materials, 2002, 14, No. 23, pp. 1728-1732) have described a pneumatic mechanism using carbon nanotubes in which aqueous electrochemistry is used to generate gas within a confined space allowing for mechanical motion. In each of these non-faradaic approaches, the load-bearing actuation materials are inherently a gaseous or liquid phase and may be expected to have low elastic modulus and consequently low actuation energy density and actuation stress, compared to the approach of the present invention.
With respect to solid-state electrochemistry, it is well-known to those skilled in the art of solid state intercalation compounds, for instance, those working in the battery field, that certain compounds undergo expansion or contraction as their chemical composition is electrochemically altered by ion insertion or removal (faradaic processes). K. Takada and S. Kondo (Solid State Ionics, Vol. 53-56, pp. 339-342, 1992, and Japanese Patent Application 02248181) have further demonstrated free strain in consolidated solid compounds undergoing electrochemically induced composition change. They reported about 0.1% free strain using AgxV2O5 as a Ag intercalating compound, which is a level of strain comparable to that reached by many commercial piezoelectric materials (e.g., those based on lead-zirconium-titanate (PZT)). However, no mechanical load was provided and so mechanical work was not demonstrated despite the observation of displacement. G. Gu, M. Schmid, P.-W. Chiu, A. Minett, J. Fraysse, G.-T. Kim, S. Roth, M. Kolov, E. Munoz and R. H. Baughmann (Nature Materials, Vol. 2, pp. 316-319) have used mattes of V2O5 nanofibres for actuation using aqueous electrochemistry. In this instance, they reported strain under unloaded conditions of up 0.21%, and the production of stress under nominally zero-strain conditions of up to 5.9 MPa, although whether the process used to generate the stress was faradaic or non-faradaic was not known.